The skin plays a vital role in protecting against the external environment, and such a function of the skin is called “barrier function”. The skin barrier function is a protective function to protect against many different types of irritating stimuli from outside (e.g., chemicals, air pollutants, dryness, UV radiation, etc.) and to prevent excessive moisture loss through transcutaneous evaporation. Such a protective function can be maintained only when the stratum corneum consisting of keratinocytes is normally formed.
The outmost layer of the epidermis, stratum corneum (horny layer), is formed from keratinocytes and composed of layers of corneocytes that is the final outcome of keratinocyte maturation, and layers of lipids surrounding the corneocytes.
The corneocytes are characteristic cells derived from basal cells that continuously proliferate in the basal layer of the epidermis and undergo migration towards the skin surface whilst undergoing changes of shape and function. As a specified period of time elapses, old corneocytes are shedding from the skin surface and replaced by new ones. This process of repetitive modification is called the differentiation or keratinization of epidermal cells.
While forming the stratum corneum (horny layer) during the keratinization process, the keratinocytes also produce natural moisturizing factors (NMFs) and intercellular lipids (e.g., ceramides, cholesterols, fatty acids, etc.). This allows the stratum corneum to have solidity and plasticity and to serve a skin barrier function.
The stratum corneum is liable to lose its skin barrier function due to certain life habit factors like frequent soap washing of the skin on the face or whole body, environmental factors like dry air, air pollutants, etc., or endogenous diseases including atopic skin diseases, senile skin diseases, etc. With a growing number of factors related to this issue in modern times, the more people are suffering from the dry skin problem and the associated disturbances.
As well known in terms of the skin moisturization mechanism to maintain skin moisturization, it is very important to retain moisture in each layer of the skin by increasing the natural moisturizing factors (NMFs) that are made up of specific amino acids. The amino acids constituting the natural moisturizing factors (NMFs) are the breakdown product from the degradation of a protein called “filaggrin” that is produced by the epidermal keratinocytes. Hence, the ability to promote the production of filaggrin is of significance in the expression of the skin moisturizing function.
In addition, the keratinocytes express specific keratins at each stage of differentiation. For example, the keratinocytes in the basal layer primarily express keratin 5 (K5) and keratin 14 (K14). As the keratinocytes move towards the upper, visible layer from the basal layer, they express pairs of keratin 10 (K10) and keratin 1 (K1) in place of the K5 and K14 pairs. Loricrin, expressed late during terminal differentiation in epidermal keratinocytes, is linked to the cell membrane in the upper, granular layer to complete a protein, so it can be used as a marker for the terminal differentiation of keratinocytes.
Accordingly, there is a demand for development of a skin moisturizing preparation capable of promoting the expression of filaggrin and loricrin for forming the stratum corneum to promote the skin moisturization and strengthen the skin barrier for preventing moisture loss from the skin, in order to prevent skin ageing and maintain skin health against change of the external environment, including dry air, UV radiation and various air pollutants.